In response to the demand for increasingly complex integrated electronic circuits, System in Package circuits or SiPs have been created. Such circuits combine several circuit parts which are realized on their respective substrates and which are assembled into the same package. Each circuit part can have a different function within the complete circuit, such as an outside connection function, a random access memory function, a multimedia communication function, a digital or analog baseband function, a radio function, a power supply function, a user interface function, etc.
Within the package, the different circuit parts are commonly connected to each other by wire bonding, or by solder bumps which connect connection pads oriented to face each other on each circuit part. Wire bonding is long and difficult to implement, and only allows the creation of electrical connections at the periphery of the circuit parts. The number of wire bonding connections is therefore limited. As for solder bumps, these can be placed all over the surface of an integrated electronic circuit part, but the number of electrical connections which can be implemented in this manner is limited by the size of the bumps relative to the size of the substrate of each circuit part, with this bump size ranging from several micrometers to several tens micrometers.
Various methods have therefore been proposed for realizing smaller connections between two parts of an integrated electronic circuit, when each circuit part is created from a separate substrate and each part therefore comprises transistors and connection levels superimposed above the transistor level.
In the first of these methods, the two substrates are first assembled in parallel, one atop the other, by molecular bonding or by use of an intermediate layer of polymer, then chimneys are etched, starting from the same external side of the assembly. An electrical connection is then realized between the two circuit parts by forming a conductive bridge on said side, connecting two chimneys filled with conductive material, each of which extends to an internal electrical connection of one of the two circuit parts. To implement this, one of the two chimneys must be particularly deep in order to traverse one of the substrates and the bonding interface between the two circuit parts. The chimney must have a cross-section large enough to allow filling the entire depth of the chimney with conductive material; therefore such connections cannot be made at a significant density in the assembled circuit. In addition, the process for manufacturing such a circuit with two parts is long and complex, because of the numerous specific additional steps which are necessary to form the connections which cross the bonding interface. Lastly, the presence of a polymer bonding layer is incompatible with certain high temperature processes applied to the circuit in a later step.
In a second of these methods, protruding metal pads are shaped on the surface of the two circuit parts, and connected to their respective electronic components as specified in the circuit design. These pads are arranged to come into contact in pairs when the two circuit parts are oriented to face each other. The assembly is then realized by metal bonding between the paired pads facing each other, compressing the two circuit portions together and simultaneously applying heat of between 300° C. and 400° C. However, the required compressive force is incompatible with the use of portions with low dielectric permittivity in each circuit part, because of the fragility of such dielectric portions.
In a third and last method, copper vias reach and overflow the surface of the two circuit parts. Portions of tin are built up onto the vias of one of the two circuit parts, then brought into contact with the vias of the other circuit part while heat is applied. These tin portions thus become solders which electrically connect the two circuit parts. However, the circuit assembly obtained in this manner is fragile.